The story of the Curtiss P-60 is a rather sad one. This aircraft was a desperate attempt on the part of Curtiss to design a replacement for the venerable P-40. It went through a dizzying series of changes in engines, changes in requirements, and changes in designations, the gyrations of which are difficult to keep straight. Its ultimate failure was a reflection of changing USAAF requirements, but was also a reflection of a company which was beginning to run out of fresh new ideas.
The story of the Curtiss P-60 is quite convoluted and rather difficult to follow. Nevertheless, pour yourself a cup of coffee and follow along :-).
The convoluted history of the P-60 actually begins back with the Curtiss XP-46. Following the failure of the XP-46 to win any Army production orders, the Curtiss company proposed yet another design in their search for the eventual replacement for the P-40. This was the Curtiss Model 88, which was basically an improved XP-46 powered by the yet-to-be-built 1600-hp Continental XIV-1430-3 twelve-cylinder liquid-cooled inverted Vee engine. The Model 88 was to use the fuselage and tail assembly of the P-40D combined with a NACA laminar flow wing. Armament was to have consisted of eight wing-mounted 0.50-inch machine guns. The mainwheel retraction scheme reverted to the sequence used by the original P-40, with the mainwheels rotating 90 degrees before they retracted rearwards into wing wells. Maximum speed was projected to be 430 mph.
On October 1, 1940, the USAAC ordered two examples of the Model 88 under the designation XP-53. Serials were 41-140 and 41-19508. However, in a conference held six weeks later, the USAAC informed Curtiss-Wright of its need for a fighter combining laminar flow wing technology with the British Rolls-Royce Merlin engine. Since the XP-53 was already being designed for laminar-flow wings, Curtiss proposed to convert the second XP-53 airframe (41-19508) to the Merlin engine while it was undergoing construction. This airframe was redesignated Model 90 by the company. The USAAC accepted this idea, and assigned the designation XP-60 to the new aircraft. The XP-60 was to take the 1300-hp Packard-built Rolls Royce Merlin V-1650-1 engine as used in the XP-40F then under development. The other XP-53 airframe was to retain the Continental engine.
However, while the XP-53 and XP-60 were both undergoing construction, the Army cancelled the XP-53 order because of the excessive delays in the temperamental Continental XIV-1430 engine. The XP-53 never flew. As it turned out, the Continental engine never did enter production, and all of those aircraft projects which had planned for it ultimately failed.
In November 1941, the XP-53 airframe was converted into a static test airframe in support of the P-60 project, and its bullet-proof windshield, self-sealing fuel tanks, and armament were scavenged and transferred to the XP-60.
During construction of the XP-60 aircraft, it was decided to replace the rearward- retracting P-40 style landing gear with a new inward-retracting gear similar to that which had been fitted to the abortive XP-46. Initially, the XP-60 was fitted with a British-built Merlin 28 engine. The Model 90 (XP-60) flew for the first time on September 18, 1941, only eleven days before the first flight of the disappointing XP-46. The performance of the XP-60 was disappointing as well, with a top speed of only 387 mph at 22,000 feet. It took 7.3 minutes to reach an altitude of 15,000 feet, and service ceiling of 29,000 feet. Some of the reason for the disappointing performance was due to the wing surface not being finished to the degree of smoothness required for the laminar flow wing. Another factor was the fact that the Merlin engine did not deliver the guaranteed output. Empty weight was 7008 pounds, gross weight was 9277 pounds, and maximum takeoff weight was 9700 pounds. Dimensions were wingspan 45 feet 5 1/4 inches, length 33 feet 7 1/2 inches, height 12 feet 4 inches, and wing area 275 square feet.
On two occasions, the XP-60 had suffered damage as the result of undercarriage failures. During the test flights, it was found necessary to enlarge the vertical and make some minor modifications. These resulted in a change in company designation to Model 90A.
The XP-60 (41-19508) was modified in August of 1942 with the installation of a Packard Merlin V-1650-3 (license-built Merlin 61) of 1350 hp with a two stage supercharger. A four-bladed propeller was fitted. The aircraft was redesignated XP-60D by the Army and Model 90B by the factory. By the time that the changes were made, the intervening Army designations B and C had been assigned to other improved versions of the XP-60. The XP-60D (41-19508) was destroyed in a crash on May 6, 1943.
In late 1941, concern was expressed that the license-built Merlin engine would be in such great demand for other aircraft that there would likely be engine shortages which would delay the P-60 program. Consequently, consideration was given to alternative powerplants for the P-60. The liquid-cooled Allison V-1710 engine was selected on the basis of reliability and availability. On October 21, 1941, a contract for 1950 P-60As was approved. The turbosupercharged Allison V-1710-75 liquid-cooled engine offering 1425 hp at 25,000 feet was specified as the powerplant.
However, on November 17, 1941, it was concluded that the P-60A would be underpowered if the Allison engine were used, and that either a more powerful engine should be found or else another fighter be built instead of the P-60A.
Following Pearl Harbor, officials had second thoughts about the desirability of interrupting P-40 production at such a critical point by the introduction of a completely new type. On December 20, 1941, work on the P-60A project was ordered halted, and on January 2, 1942, the production order for the 1950 P-60A fighters was officially cancelled in favor of more P-40Ks and Ls plus some Curtiss-built P-47G Thunderbolts.
However, the P-60 program was not to be scrubbed completely. The Army decided that three experimental P-60s should be built--one XP-60A, one XP-60B, and one XP-60C. The XP-60A (serial number 42-79423) was to have the Allison V-1701-75 engine and a General Electric B-14 turbo- supercharger. The XP-60B (serial number 42-79425) was to be similarly powered, but was to use the Wright SU-504-2 turbosupercharger. The XP-60C (serial number 42-79424) was to use the experimental Chrysler XIV-2220 sixteen-cylinder engine. Normally, a switch in engines was considered by the Army as calling for change of model number as well, but such was not the case here.
The first of these new experimental P-60 aircraft was the XP-60A. The Allison-powered XP-60A could be considered as an adaptation of the XP-60 wing to a new fuselage and a new powerplant, in much the same spirit as the XP-60 could be regarded as a P-40D with a new wing. The XP-60A was given the company designation of Model 95A (Model 95 having been a design study which had been discontinued). Nose and fuselage contours of the XP-60A (serial no 42-79423) were extensively revised to accommodate the Allison engine, and armament was reduced to six 0.50-inch guns in the wings. A four-bladed propeller was fitted.
The XP-60A made its initial ground taxiing tests in late October of 1942. However, during one of these tests, a minor fire occurred in the engine due to the lack of cooling air in the shrouds surrounding the exhaust manifold. The turbosupercharger and long exhaust manifold were therefore removed from the aircraft, and short exhaust stacks were substituted. The XP-60A (42-79423) flew for the first time in this form on November 11, 1942. Empty weight was 7806 pounds, gross weight was 9616 pounds, and maximum takeoff weight was 10,160 pounds. Dimensions were wingspan 41 feet 3 3/4 inches, length 33 feet 7 1/2 inches, height 12 feet 4 inches, and wing area 275 square feet. Estimated maximum speed (never achieved in tests) was 420 mph at 29,000 feet and 324 mph at sea level. It was estimated that an altitude of 15,000 feet could be attained in 6.5 minutes. Service ceiling was 35,200 feet. The maximum speed (especially at low altitudes) and the initial climb rate were rather disappointing. The XP-60A aircraft was soon dismantled and some of its parts were used in the later XP-60C and XP-60E.
The poor performance of the XP-60A had caused official interest in the P-60 fighter to sour, the project being in serious danger of being cancelled outright. However, Curtiss-Wright proposed to the Army that the XP-60C prototype then under construction be fitted with the Pratt & Whitney R-2800 radial air-cooled engine driving a pair of three-bladed contrarotating propellers. The substantial improvement in performance that this modification promised to provide aroused sufficient interest that in November 1942 the Army issued a letter contract for the production of five hundred R-2800-powered production P-60A-1-CU fighters. The first 26 aircraft on this production contract, Army serials 43-32762/32787, were to be delivered as service test models designated YP-60A.
The XP-60C (Model 95C, Army serial number 42-79424) was originally to have had an airframe similar to that of the XP-60A and XP-60B, but fitted with the new and experimental 2300 hp Chrysler XIV-2220 engine. Since this engine was experiencing development difficulties, an order was given in September 1942 to complete this aircraft with a 2000-hp Pratt & Whitney R-2800-53 engine driving a a pair of three-bladed contrarotating propellers. Armament was reduced to four 0.50-inch machine guns. First flight of the XP-60C was on January 27, 1943. Apart from somewhat high elevator and rudder forces, the aircraft's flight characteristics were generally satisfactory.
The single XP-60B (Model 95B, Army serial number 42-79425) was to have been similar to the XP-60A but with a Wright instead of a General Electric supercharger for the V-1710-75 engine. The aircraft was never completed in this configuration. On December 2, 1942, the Army ordered that it be fitted with a Pratt & Whitney R-2800-10 radial engine in place of the original V-1710. Unlike the XP-60C, a single-rotation four-bladed propeller was to be used. In this form, the aircraft was redesignated XP-60E. Owing to the lighter propeller installation of the XP-60E as compared to that of the XP-60C, it was found necessary to move the R-2800 engine forward by ten inches. Owing to a fault experienced during the initial ground running tests which necessitated a change of engines, the first flight of the XP-60E did not take place until May 26, 1943.
During the latter part of April 1943, the USAAF decided to undertake a series of comparative tests at Patterson Field with various fighter types in an attempt to weed out the least desirable types so that it could concentrate on the best types. Curtiss-Wright was notified by the Army that the XP-60E would have to be delivered for tests within four days. Since the XP-60E had not yet made its first flight, Curtiss-Wright decided to substitute the XP-60C in its place. The XP-60C was hastily reassembled and delivered to Patterson Field.
During trials with the XP-60C at Patterson Field, it proved impossible to obtain full rated power. In addition, the experimental wing finish had peeled off from the leading edge of the wing, destroying the smooth laminar-flow characteristics and resulting in a further loss of speed. Consequently, the XP-60C made a very poor impression on the Army, being in fact inferior to the Republic P-47D and the North American P-51B. The P-60 series was henceforth eliminated from any further consideration for production. In June 1943 the Army contract for the P-60A-1-CU was reduced from 500 to just two aircraft.
The XP-60C was powered by a Pratt & Whitney R-2800-53 engine delivering 2000 hp. Empty weight was 8698 pounds, gross weight was 10,785 pounds, and maximum takeoff weight was 11,835 pounds. Dimensions were wingspan 41 feet 3 3/4 inches, length 34 feet 1 inches, height 12 feet 4 inches, and wing area 275 square feet. Maximum speed was 414 mph at 20,350, 324 mph at sea level. An altitude of 30,000 feet could be attained in 6 minutes, and initial climb rate was 3890 feet per minute. Service ceiling was 37,900 feet. Normal range was 315 miles. Armament consisted of four 0.50-inch machine guns with 300 rpg mounted in the wings.
Following the return of the XP-60C to Curtiss-Wright, some further tests were undertaken, but a forced landing terminated all testing work with this aircraft.
By this time, there was essentially no chance for the P-60, since the P-47 and P-51 seemed to satisfy all the Army's needs for fighters. Nevertheless, the Army agreed to test the delayed XP-60E which had missed out on the May 1943 trials at Patterson Field. In January 1944, the XP-60E (Model 95D) was flown to Elgin Field for official tests. The engine was a Pratt & Whitney R2800-10 eighteen-cylinder radial offering 2000 hp. Empty weight was 8285 pounds, gross weight was 10,320 pounds, and maximum takeoff weight was 11,520 pounds. Dimensions were wingspan 41 feet 3 3/4 inches, length 33 feet 11 inches, height 12 feet 6 inches, and wing area 275 square feet. Maximum speed was 410 mph at 20,200, 391 mph at 24,200 feet, and 405 mph at 15,000 feet. An altitude of 15,000 feet could be attained in 4.8 minutes. Service ceiling was 38,000 feet. Normal range was 315 miles. Armament consisted of four 0.50-inch machine guns with 250 rpg mounted in the wings. USAAF test pilots found that the XP-60E did not compare very favorably in level flight performance with later fighters, but it did match them in climbing rate. The aircraft was sensitive to slight changes in flight condition and had to be constantly retrimmed. Stability in level flight was poor and the climing speed was difficult to maintain.
In May of 1944, Curtiss-Wright finally recognized that the P-60 was a lost cause, and indicated to the Army that they wanted to discontinue all further work on the project. However, the USAAF insisted that the company follow through on its agreement and complete at least one of the two YP-60A aircraft still under construction under the revised P-60A-1-CU contract. These aircraft had been redesignated YP-60E owing to the number of design modifications incorporated that were related to the XP-60E.
One of the YP-60As was to see the light of day as a YP-60E. This was the second YP-60A, serialled 43-32763. It flew for the first time on July 15, 1944, powered by a 2100 hp Pratt & Whitney R-2800-18 eighteen-cylinder radial engine driving a single four-bladed propeller. It differed from previous P-60s in having a bubble canopy over the cockpit and revised fuselage and vertical tail shapes, so that it ended up looking a lot like a P-47D-25 Thunderbolt. Empty weight of the YP-60E was 8225 pounds, gross weight was 10,270 pounds, and maximum takeoff weight was 11,520 pounds. Dimensions were wingspan 41 feet 3 3/4 inches, length 33 feet 11 inches, height 12 feet 6 inches, and wing area 275 square feet. Estimated maximum speed was 405 mph at 24,500. Initial climb rate was estimated at 4200 feet per minute. Service ceiling was 34,000 feet. Armament consisted of six 0.50-inch machine guns with 267 rpg mounted in the wings.
Only two test flights of the YP-60E were undertaken by Curtiss-Wright before the aircraft was transferred to Wright Field. By this time, the Army had absolutely no need for the P-60, and no further trials were undertaken. The YP-60E was eventually disposed of as surplus after the war. It was purchased by James DeSanto and was entered in the 1947 National Air Races with Race No. 80 and civil registration NX21979, but crashed on a qualifying flight.
Here is a brief summary of serial numbers, which may help to make the
history of the P-60 a bit less confusing :-)
41-140 Curtiss XP-53 - cancelled before construction 41-19508 Curtiss XP-53 ---> XP-60 ---> XP-60D 42-79423 Curtiss XP-60A 42-79424 Curtiss XP-60C 42-79425 Curtiss XP-60B --> XP-60E 43-32762/32787 Curtiss YP-60A 32763 to YP-60E, rest cancelled. 43-32789/33262 Curtiss P-60A-1-CU - all cancelled June 1943.